This invention relates to DC linear actuators. There are basically two types of linear actuators, the most familiar being a ferromagnetic clad DC solenoid having a ferromagnetic core surrounded by a fixed coil within a ferromagnetic housing. The core is drawn into the coil and approaches and usually contacts a fixed stop or pole piece projecting into the coil opening from one end of the coil. When the coil is energized the armature or core is drawn into the coil and does the desired work. The force exerted on the armature increases as the air gap between the armature and the fixed pole piece decreases. Such solenoids are useful where the load is moved over a fixed distance or stroke, but become less attractive when the stroke must be varied. At that point, the solenoid cannot be used effectively.
The other kind of DC linear actuator is called a moving coil linear actuator which has a magnet fixed in a ferromagnetic shell and a ferromagnetic core. A movable coil is situated between the magnet and the core and moves relative to the magnet as the current in the coil is varied. Coil movement results in requiring very flexible lead wires or brushes to connect the coil to a DC electrical source. This can be costly and results in mechanical losses (or resistance to movement) which affects response of the coil. In order to get a long stroke, it is necessary to use long magnets and these are expensive. The mechanical work is accomplished by axial coil movement and the mass of the coil affects the output force and response time with the result that some applications limit the physical size of the coil and this in turn places restrictions on heat dissipation and limits the power handling capability of the coil and the structure. The advantage of the moving coil, however, is that it has a reasonably uniform force over the stroke of the coil. This is in contrast to the rapid increase in force on the solenoid type actuator as it approaches the fixed pole piece (i.e., as the air gap decreases).